Comprehensive analysis of photon dynamics in thin-film GaAs solar cells with planar and textured rear mirrors

Abstract

This study describes a comprehensive analysis of light trapping and photon recycling (PR) in thin-film GaAs solar cells, explicitly considering the effects of (non-perfect) light scattering at a textured rear mirror. We calculate the probabilities of escape, reabsorption in the absorber and parasitic absorption for internally generated photons, as well as the reflectance, absorptance and parasitic absorptance of externally incident photons. Combined with the internal luminescent efficiency to account for non-radiative recombination non-explicitly, but in a phenomenological manner, device performance metrics such as the short-circuit current density ( J s c ), open-circuit voltage ( V o c ) and its radiative limit, the external luminescent efficiency, the contribution of PR to the V o c and the power conversion efficiency ( P C E ) are derived. We analyze a typical thin-film GaAs solar cell architecture, consisting of an n-GaAs/p-InGaP heterojunction structure cladded by commonly employed front- and rear-side layer stacks. The probabilities of escape, reabsorption and parasitic absorption and the resulting performance metrics are presented as a function of absorber thickness, rear-mirror reflectivity, haze factor, internal luminescent efficiency, front grid coverage and band tailing. The simulations show that the implementation of a textured rear mirror reduces PR and its importance for device performance. In these textured cells, a high rear-mirror reflectivity and, thereby, efficient PR, is less crucial to achieve V o c values close to the radiative limit. The increased J s c in textured cells renders light trapping favorable for the P C E , regardless of absorber thickness and material quality and despite reduced PR. Furthermore, the impact of an absorbing front grid and lateral transport of internal luminescence on PR is estimated, emphasizing the importance of transparent (non-absorbing) contact layers, at the rear, but importantly, also at the front of the device. To conclude, P C E limits of 300-nm-thick GaAs solar cells are deduced in the context of an experimentally realized light-trapping scheme based on a randomly textured rear mirror. P C E s exceeding 25.5% are deemed realistic. • Photon recycling and light trapping are studied in textured thin-film GaAs cells • Ray-optical modeling yields probabilities of escape and (parasitic) absorption • Textured rear mirrors reduce photon recycling and its impact on cell performance • Light trapping improves efficiency for any absorber thickness and material quality • Efficiency limits of 300-nm GaAs cells are currently 25.5% and ultimately ¿28%